7,540 research outputs found
Post-Newtonian gravitational radiation and equations of motion via direct integration of the relaxed Einstein equations. V. Evidence for the strong equivalence principle to second post-Newtonian order
Using post-Newtonian equations of motion for fluid bodies valid to the second
post-Newtonian order, we derive the equations of motion for binary systems with
finite-sized, non-spinning but arbitrarily shaped bodies. In particular we
study the contributions of the internal structure of the bodies (such as
self-gravity) that would diverge if the size of the bodies were to shrink to
zero. Using a set of virial relations accurate to the first post-Newtonian
order that reflect the stationarity of each body, and redefining the masses to
include 1PN and 2PN self-gravity terms, we demonstrate the complete
cancellation of a class of potentially divergent, structure-dependent terms
that scale as s^{-1} and s^{-5/2}, where s is the characteristic size of the
bodies. This is further evidence of the Strong Equivalence Principle, and
supports the use of post-Newtonian approximations to derive equations of motion
for strong-field bodies such as neutron stars and black holes. This extends
earlier work done by Kopeikin.Comment: 14 pages, submitted to Phys. Rev. D; small changes to coincide with
published versio
Electrostatics in the Stability and Misfolding of the Prion Protein: Salt Bridges, Self-Energy, and Solvation
Using a recently developed mesoscopic theory of protein dielectrics, we have
calculated the salt bridge energies, total residue electrostatic potential
energies, and transfer energies into a low dielectric amyloid-like phase for 12
species and mutants of the prion protein. Salt bridges and self energies play
key roles in stabilizing secondary and tertiary structural elements of the
prion protein. The total electrostatic potential energy of each residue was
found to be invariably stabilizing. Residues frequently found to be mutated in
familial prion disease were among those with the largest electrostatic
energies. The large barrier to charged group desolvation imposes regional
constraints on involvement of the prion protein in an amyloid aggregate,
resulting in an electrostatic amyloid recruitment profile that favours regions
of sequence between alpha helix 1 and beta strand 2, the middles of helices 2
and 3, and the region N-terminal to alpha helix 1. We found that the
stabilization due to salt bridges is minimal among the proteins studied for
disease-susceptible human mutants of prion protein
Napanee : A Song Founded On Actual Facts
https://digitalcommons.library.umaine.edu/mmb-vp/4801/thumbnail.jp
Testing Alternative Theories of Gravity using LISA
We investigate the possible bounds which could be placed on alternative
theories of gravity using gravitational wave detection from inspiralling
compact binaries with the proposed LISA space interferometer. Specifically, we
estimate lower bounds on the coupling parameter \omega of scalar-tensor
theories of the Brans-Dicke type and on the Compton wavelength of the graviton
\lambda_g in hypothetical massive graviton theories. In these theories,
modifications of the gravitational radiation damping formulae or of the
propagation of the waves translate into a change in the phase evolution of the
observed gravitational waveform. We obtain the bounds through the technique of
matched filtering, employing the LISA Sensitivity Curve Generator (SCG),
available online. For a neutron star inspiralling into a 10^3 M_sun black hole
in the Virgo Cluster, in a two-year integration, we find a lower bound \omega >
3 * 10^5. For lower-mass black holes, the bound could be as large as 2 * 10^6.
The bound is independent of LISA arm length, but is inversely proportional to
the LISA position noise error. Lower bounds on the graviton Compton wavelength
ranging from 10^15 km to 5 * 10^16 km can be obtained from one-year
observations of massive binary black hole inspirals at cosmological distances
(3 Gpc), for masses ranging from 10^4 to 10^7 M_sun. For the highest-mass
systems (10^7 M_sun), the bound is proportional to (LISA arm length)^{1/2} and
to (LISA acceleration noise)^{-1/2}. For the others, the bound is independent
of these parameters because of the dominance of white-dwarf confusion noise in
the relevant part of the frequency spectrum. These bounds improve and extend
earlier work which used analytic formulae for the noise curves.Comment: 16 pages, 9 figures, submitted to Classical & Quantum Gravit
The post-Newtonian limit in C-theories of gravitation
C-theory provides a unified framework to study metric, metric-affine and more
general theories of gravity. In the vacuum weak-field limit of these theories,
the parameterized post-Newtonian (PPN) parameters and can
differ from their general relativistic values. However, there are several
classes of models featuring long-distance modifications of gravity but
nevertheless passing the Solar system tests. Here it is shown how to compute
the PPN parameters in C-theories and also in nonminimally coupled curvature
theories, correcting previous results in the literature for the latter.Comment: 5 pages, no figures; To appear in PRD as a rapid communicatio
Symmetron Fields: Screening Long-Range Forces Through Local Symmetry Restoration
We present a screening mechanism that allows a scalar field to mediate a long
range (~Mpc) force of gravitational strength in the cosmos while satisfying
local tests of gravity. The mechanism hinges on local symmetry restoration in
the presence of matter. In regions of sufficiently high matter density, the
field is drawn towards \phi = 0 where its coupling to matter vanishes and the
\phi-> -\phi symmetry is restored. In regions of low density, however, the
symmetry is spontaneously broken, and the field couples to matter with
gravitational strength. We predict deviations from general relativity in the
solar system that are within reach of next-generation experiments, as well as
astrophysically observable violations of the equivalence principle. The model
can be distinguished experimentally from Brans-Dicke gravity, chameleon
theories and brane-world modifications of gravity.Comment: 4 pages. v3: version appearing in PR
Solar system and equivalence principle constraints on f(R) gravity by chameleon approach
We study constraints on f(R) dark energy models from solar system experiments
combined with experiments on the violation of equivalence principle. When the
mass of an equivalent scalar field degree of freedom is heavy in a region with
high density, a spherically symmetric body has a thin-shell so that an
effective coupling of the fifth force is suppressed through a chameleon
mechanism. We place experimental bounds on the cosmologically viable models
recently proposed in literature which have an asymptotic form f(R)=R-lambda R_c
[1-(R_c/R)^{2n}] in the regime R >> R_c. From the solar-system constraints on
the post-Newtonian parameter gamma, we derive the bound n>0.5, whereas the
constraints from the violations of weak and strong equivalence principles give
the bound n>0.9. This allows a possibility to find the deviation from the
LambdaCDM cosmological model. For the model f(R)=R-lambda R_c(R/R_c)^p with
0<p<1 the severest constraint is found to be p<10^{-10}, which shows that this
model is hardly distinguishable from the LambdaCDM cosmology.Comment: 5 pages, no figures, version to appear in Physical Review
Probing the Brans-Dicke Gravitational Field by Cerenkov Radiation
The possibility that a charged particle propagating in a gravitational field
described by Brans-Dicke theory of gravity could emit Cerenkov radiation is
explored. This process is kinematically allowed depending on parameters
occurring in the theory. The Cerenkov effect disappears as the BD parameter
omega tends to inftinity, i.e. in the limit in which the Einstein theory is
recovered, giving a signature to probe the validity of the Brans-Dicke theory.Comment: 8 pages, no figure
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